The present invention relates to a visual display apparatus for displaying an enlarged image of an original document such as a microfilm frame.
Various types of optical projectors are currently in widespread use for projecting enlarged images of original documents such as microfilm frames onto a viewing screen. Slide overhead and opaque projectors are typical examples and all operate on the same principle. The original document is illuminated by a light source and an optical system comprising a converging lens focusses an optical image of the document onto a viewing screen.
All of these prior art projectors suffer from the same drawback in that in order to provide an optical image of sufficient brightness for satisfactory viewing the room in which the viewing is being done must be darkened. Whereas systems employing the rear screen projection principle allow viewing in lighted rooms, the size of the image is limited to such an extent that only one or a very few persons can comfortably view the image.
Another major drawback of optical projectors which project an image onto a viewing screen in that the original document is heated by the illumination lamp to such an extent that it may be damaged. This problem is particularly acute in the projection of plastic transparencies, and limits the size and brightness of the projected image. Prolonged viewing increases the possibility of heat damage. Decreasing the intensity of the illumination lamp in order to preclude heat damage of course reduces the intensity of the projected image and further limits the size thereof.